The present invention relates to an improvement in a hydraulic control system for automatic transmission with 4 forward speeds and it aims to prevent shocks caused by shifting down from the 4th speed of over-drive to the directly connected 3rd speed.
There is a growing demand in recent automobile industry for higher efficiency and less energy consumption not only in the engine but also in other various devices such as the automatic transmission, etc. An automatic transmission system with 4 forward speeds including the over-drive has been introduced to meet such demands, which enables automatic shifting in correspondence with the driving conditions of a vehicle by means of a device for controlling the oil pressure.
An example of such automatic transmission system is provided with a 4th speed clutch which connects the input shaft (an output shaft of a torque converter) with a carrier of a ravigneawx type planetary gear set to achieve the over-drive. In this system, the oil pressure is so controlled that the 4th speed clutch is engaged with one of the brakes, i.e. a kick-down brake when the 4th speed is obtained, whereas when the 3rd speed is obtained, two other different clutches, i.e. a front clutch and a rear clutch, are engaged. In obtaining the 3rd speed, the 4th speed clutch is engaged in advance to prevent shocks at the time of upward shifting.
In the automatic transmission gear system of this construction, however, it is necessary to newly engage the front clutch and the rear clutch after at least the kick-down brake has been released when shifting down from the 4th to the 3rd speed and in doing so timely engagement of the rear clutch which has a greater torque transmitting capacity is especially difficult, and causes shifting shocks.